After reading and re-reading the relevant section of the Curator Guide, it seems to me the most important aspect of names which are added to iNaturalist is that they should be useful for observers and identifiers on iNaturalist.
Whether names are in use elsewhere or not is a secondary concern - the Guide spends six paragraphs discussing overly generic names and the problems they cause, going as far as to say “If you find names that don’t meet these criteria, delete them without mercy.”, and only one paragraph to tell curators not to make up names.
So: I think it’s a reasonable interpretation of what the Curator Guide says, to say that names which make it harder for iNaturalist observers and identifiers to find the correct species, for whatever reason, should be deleted regardless of whether they’re in use or not. The names on iNaturalist are not there primarily to document existing usage. If the name is in use and it doesn’t cause problems, then it likely helps iNaturalist observers and identifiers find the correct species, and that is the reason the name is added to iNaturalist, not simply that it’s in use.
Thoughts?